Purification of pyrazine



May 8, 1962 E. C. BRITTON ETAL PURIFICATION OF' PYRAZINE Filed Oct. 13, 1954 TTOR/VEYS tte tts dice 3,033,864 PURIFICATIUN F PYRAZDJE This invention relates to the purification of pyrazines and piperazines. More particularly, it pertains to a method for the separation of alkanolamines, from mixtures thereof with at least one heterocyclic nitrogen compound selected from the class consisting of pyrazines and piperazines, by azeotropic distillation.

Pyrazines :and piperazines can be prepared from alkanolamines, e.g. by heating, usually in the presence of a hydrogenation catalyst. Bain and Pollard, in J. Am. Chem. Soc. 61, 532 (1939), report having formed several piperazine derivatives, such as 2,5-dimethyl piperazine, by heating solutions of the corresponding valkanolamines, such as isopropanolarnine, in dioxane together with copper chromite as a catalyst and in contact with hydrogen at superatmospheric pressure, to temperatures in the order of from 250 to 275 C. Pyrazines can be formed by dehydrogenation of the corresponding piperazines.

It is often ditcult to separate the desired reaction product from the unreacted alkanolamine by ordinary methods such as distillation. Even when the desired product can be partially separated from the alkanolamine, itis sometimes necessary or desirable to eliect a higher degree of purification of the pyrazine or piperazine than is ordinarily obtainable by the usual methods.

It is an object of this invention to provide a method for the purification of pyrazines and piperazines obtained from alkanolamines.

A further object is to provide such a method for the separation of mixtures comprising pyrazines and/or piperazines and alkanolamines.

Another object is to provide such a method whereby pyrazines and piperazines can be substantially freed of alkanolamines.

`Another object is to provide such a method whereby alkanolamines can be recovered from mixtures Vwith pyrazines and/or piperazines in a form satisfactory for further use.

Other objects and advantages will be evident from the following description.

The objects of this invention are attained in a method wherein mixtures of pyrazines and/or piperazines and alkanolamines are fractionally distilled together with a codistillation agent whereby lower-boiling azeotropes of the codistillation agent and the alkanolamines are separated as distillates from residues which are enriched in respect to the pyrazines 'and/or piperazines.

The invention can be practiced with any codistillation agent which is non-reactive chemically with the components of the starting material underdistillation conditions, and which forms with the alkanolamine in the starting material a distillate whose boiling point is below the boiling points of other components of the distillation mixture.

It is desirable that the azeotropic distillate contain appreciable proportions of both the alkanolamine and the codistillation agent. To this end, there are usually employed codistillation agents having normal boiling points, i.e., boiling points at one atmosphere pressure, between about '130 and 200 C., preferably between 130 and,

165 C. Usually the preferred codistillation agents have normal boiling points below that of the alkanolamine to be separated since theseare generally more eicient than are the higher boiling agents.

Suitable codistillation agents which can be employed in the practice of this invention are aliphatic hydrocarbons, aromatic hydrocarbons and nuclear chlorinated aromatic hydrocarbons having normal boiling points be-A tween about l30 and 200 C. Representative examples of such suitable codistillation agents are aliphatic hydrocarbons, particularly octane and higher hydrocarbons, including mixtures such as petroleum fractions, cycloali-v phatic hydrocarbons such as ethyl cyclohexane, aromatic hydrocarbons such as ethylbenzene, the xylenes, diethylbenzene, ethyltoluene and cumene, and nuclear chlorinated aromatic hydrocarbons such -as chlorobenzene.

Representative examples of mixtures from which the alkanolamine can be separated by the present method are mixtures containing piperazine and ethanolamine; pyrazine and ethanolamine; cis-2,5-dimethylpiperazine and isopropanolamine; trans-2,5dimethylpiperazine and isopropanolamine; 2,5 dimethylpyrazine and isopropanolamine; and diethylpiperazine andbutanolamine.

In practice of the invention, a mixture containing a piperazine, or :a pyrazine, or both, and an alkanolamine,

together with one of the aforementioned codistillation agents, is fractionally distilled. The distillation can be carried out in batch or in continuousvmanner and at above or below atmospheric pressure. When batch operations are employed, the codistillation agent can be charged with the mixture to be separated and in proportions necessary to form a codistillate with all of the alkanolamine. Alternatively, a smaller proportion of codistillation agent can be charged initially and additional amounts of such agent can be added gradually, eg., in small portions, to the still during the distillation. Another procedure is to separate the codistillation agent from the codistillate and to return the separated codistillation agent as reilux to the distillation zone. By the latter procedure, a relatively small proportion of codistillation agent can be continuously recycled to effect the separation of a relatively large proportion of starting mixture of alkanolamine and heterocyclic nitrogen compound. ln continuous operation, a starting mixture of alkanolamine and heterocyclic nitrogen'compound can befed continuously together with a codistillation agent, either admixed or separately, to a continuous fractional distillation zone.`

If an inventory of codistillation agent is constantly main-l tained in the distillation zone by continuously Y.separating the codistillation agent from the codistillate and vcontinuously returning the separated agent as reflux to the distillation zone, then the feed may consist ofthe alkanol-Y amine and isopropa'n'olamine starting mixture and only' Ymake-up amounts of codistillation agent need besupplied to the still. In such instance, a relatively small proportion of the agent sutlices to effect the separation of a relaltively large proportion of starting mixture of alkanolamine and heterocyclic nitrogen compound.

By means of one of these codistillation procedures, there are, obtained a distillatecomprising the alkanolamine and the codistillation agent, and a residue, comkprising the heterocyvcle nitrogenv compound, Vi.e., the

pperazine and/ofthe PYazine". By employing eicient fractional distillation, the distillate can be substantiallyV free Vof the heterocyclic nitrogen compounds and the residue can be substantially free of the alkanolamine.

- The alkanolamine can be recovered from admixture with the codistillation agent in the distillate in usual ways.Y

In some instances the alkanolamine is irnmiscible with the codistillation agent and theliquidlayers'can be mechanically separated. For example, isopropanolamine codistills with ethylcyclohexane and two layersform in the condensed codistillate, one layer being essentially isopropanolamine and the other beingy essentially ethylcyclo hexane. In other instances, where the Yalkanolamine and Y 2,5-dimethylpiperazines c ected by addinga small proportion of water to the distlllate. yFor example, a homogeneous distillateV of iso- Apropanolamine and xylene iscaused to form two layer-s by the addition of la small proportion of Water, one of the resulting layers consisting essentially of xylene and the other consisting essentially of isopropanolamine and water. After separation `from the codistillat'e, the codis- Y tillation `agent can be returned to the distillation zone to effect a vfurther codistillation with alkanolamine.

` The present method is especially advantageous for the the codistillation agent are miscible, but the codistillation agent is not miscible With water, a separation can he separation of isopropanolamine from admixture with 2,5-

dimethylpiperazines. As shown by FIGURES l and2 of the drawing, isopropanolamine normally forms an azeotrope with cis-2,5-dimethylpiperazine and with trans- 2,5dimethylpiperazine. FIGURES l and 2 show liquid- Y vapor equilibrium compositions byweight for the system isopropanolamine andthe 2,5-dimethylpiperazines. The straightA diagonal lines represent theoretically identical compositions in liquid'and Vapor phases. The S-shaped curves clinging close to the diagonals are drawn from experimental data andrshow the actual composition of the vapor 1n equilibrium with the liquid at the atmospheric that the" isopropanolamine is' slightly more volatile than the piper'azine compound. For high concentrations kkof isopropanolamine in the liquid, the curves show that'the piperazine compound is slightly `more volatile than 'the 'isopropanolamine -Whe're the curves cross the diagonal,

i.e. at about 46 percent isopropanolamine in the cis-2,5- dimethylpiperazine mixtureY and `at about 52 percent isopropanolamine in the trans-2,Sdimethylpiperazine mixture, an azeotropic composition is indicated. As is indicated by these curves, fractional distillation of mixtures of isopropanolamine and 2,5-dimethylpiperazines are not only nefcent, but the distillate is an azeotropic composition which cannot be separated byzordinary distillation.

FIGURES l and 2 Valso show the equilibrium compositions for theV system isopropanolamine and the 2,5di- -methylpiperazines in the presence `of an excess of xylene at the atmospheric boiling point ofthe mixtures.VV The compositions represented by the Vupper curves obtained in presence of xylene are computed on a solvent-free basis, Y ,i.e: the compositions of liquid and vapor phases in equilibrium at the normal boiling point in presence of xylene Iare computed on` the basis of therelative proportions of isopropanolamine andthe 2,5-dimethylpiperazines in the l .mixtures As these curves indicateyinthe presence of xylene, Ythe isopropanolamine 'ismore Volatile Ythan the throughout' the composition range. Furthermore, the Wide separation of the composition curves in presence of xylene from the straight line diagonals indicates, thatj anetlicientY separation of iso-` propanolamine from eithe'rrciso1' trans-2,5=dimethyl the .presence of Xylene. l

In some instances, a mixture of analkanolamine and a piperazine or a pyrazinevdoes not form an azeotrope.V

For example, the liquid-vapor equilibrium data for mixtures of ethanolamine and piperazine are shown inT-`IGV UREB of the drawing. The'lower Ycurveshovvs that the mixturecof ethanolamine and piperazine is non-azeotropic a-zine thancan beireadilyeiected by ordinary fractional distillation.. In such instances, the present method is'ad vantageously employedi As shown by FIGURE 3, Ydisrtillation of Vmixtures of ethanolaminefand piperazinev in Y the Vpresence of xylene makes the ethar'iolamineV more vvolatile *than the piperazine. The method is especially advantageous `for removing the lastrtraces ofY ethanolamine from mixtures rich in piperazine.

In general, the greater the diierence there is between the Vnormal boiling point of a lower-boiling codistillation agent and the normal boiling point of a Vhigher-boiling mixture of alkanolamine and heterocyclic nitrogen cornpound to be separated, the more eicient is that separation, i.e.,`the more completely and readily can the alkanolamine be separated from the piperazine or pyrazine compound by codistillation with that agent. However, the greater that diierence in normal lboiling points, the greater is the proportion of codistillation agent in the distillate and, conversely, the less the proportion of alk-anolamine. Use of a'lower-boiling codistillation agent requires that a larger proportion of codistillation agent be distilled, condensed and separated in order to elect the removal of a given amountrof alkanolamine. For example, in the distillation of a mixture of isopropanolamine and 2,5-dimethylpiperazinein the presence of cumene,V using a moderate degreegof fractionation, a larger proportion of codistillation agent is present in the distillate than when asimilar distillation is carried out with diethylbenzene as the codistillation agent. However, much less 2,5-dimethylpiperazine is found in the isopropanolarnine recovered from the distillate using cumene than when using diethylbenzene asthe codistillation agent andthe same moderate degree of fractionation.

-In general, the parailin compounds are more selective than naphthene compounds which, in turn, are more selective than aromatic compounds as codistillation agents. Inother words, the proportion of alkanolamine tovpiperazines or pyrazines in the' vapor phase ,over a given mixture in the presence of a codi'stillation agent usually varies with the chemical composition of the codistillation agent in descending order through the series of paran compounds, naphthene compounds and aromatic compounds, other factors, such as relative boiling points and distilling pressure, being equal. y

The invention can be particularly described by reference to the production ofV trans-2,5-dimethyl piperazine from isopropanolamine. Y

By a condensation of isopropanolamine with itself by heating under pressure in the presence ofA a catalyst,'there was obtained a reaction product comprising trans-2,5- Y' dimethylpiperazine, unreacted isopropanolamine and orpiperazine canrbe obtained by fractional distillation in ganic by-products. about an` equal volume of xylene and the resulting mixture was fractionally distilled. The distillate consisted largely of a mixture of xylene and isoprooanolamine, and the distillation was continued until the isopropanolamine was substantially removed from the residue. The residue, containing trans-2,S-dimethylpiperazine,V excess xylene and organic by-products, was cooled, whereupon trans-2,5-dimethylpiperazine crystallized out of the mixture and was collected by filtration in the usual manner.

The following examples show Ways in which the in- Vention'has been practiced, but should not be construed as limiting its scope.

EXAMPLE 1 This example illustrates a simple procedure for testingv a codistillation agent as to its utility and eciency in separating an alkanolamine from a mixture thereof with a piperazineor pyrazine compound.

Y To diierentportions, each weighing 670 grams, of a mixture Vof about 34 percent byweight isopropanolamine and 66 percent cis2,5-.dimethylpiperazine,.was added an equal Weight of one of the materials identied in Table I. Each of the resulting mixtures was separately brought to reflux ,at atmospheric pressure in a fractionating column 2V; inch in diameter and 12 inches long `and packed with small glass helices. At reflux equilibrium, a small fraction of 10-l`5mls. of distillate was taken and analyzed.

In Table I, foreach test the codistillation agent is iden- To this reaction product was added Yheat on the reboiler, are taken.

of the small distillate fraction recorded for each test are methylpiperazine without any codistillation agent.

As can be seen from the piperazine predominated.

codistillation agents in a simple manner.

By a similar procedure, other codistllaton agents can be shown to eifect a separation of mixtures of alkanolamines and piperazines and/ or pyrazines.

is shown, and `the analysis of the small fraction in percent by weight is shown. Also,

Ae proportions of isopropanolamine and cis-2,S-dimethylpiperazine in the small distillate fraction on a solvent-free basis. There is also shown the composition of a fraction obtained by a similar test of the same mixture of isopropanolamine and cis-2,5-di- Table I, all of the codistillation agents tested gave distillates in which the isopropanolamine predominated over the dimethylpiperazine, whereas in the absence of codistillation agent the dimethylmiscible with the alkanolamine.

A mixture comprising trans-2,5-dimethylpiperazine and about percent by Weight isopropanolamine is fed continuously to a mid-point of` an eiiicient fractionating column operating at atmospheric pressure. The column also contains ethylbenzene as a codstillation agent. The column is heated by a bottom reboiler. The vapors fractionated out at the top of the column consist of the codistillation agent and isopropanolamine and are substantially free of dimethylpiperazine. The top vapors are condensed, forming a homogeneous liquid condensate. This condensate is conducted to a short, continuous liquid- Iiquid extraction column to which a small proportion of water is also fed continuously. From the top of the extractor is withdrawn ethylbenzene, substantially free of isopropanolamine, and from the bottom of the extractor is withdrawn an aqueous isopropanolaminefraction sub- 20 stantially free of ethylbenzene. The water in the isopropanolamine fraction is removed by a separate fractional Table I Analysis of Distillate, Percent by Solvent-Free Distillate, Distilla- Weight Percent by Weight Codistillation Agent tion Temperature, C. Codstillation Isopro- Cis-2,5di Isopro- Ors-2,5-d1- Agent panolamine rnethylpanolamine methylpiperazme piperazme None 40. 4 59. 6 Ethylbenzene 73 27 Cumene 66 34 Ethylcyelohexan 75. 2 24. 8 Chlorobenzene-- 78. 4 21. 6 Diethylbenzene 61. 5 38. 5 Petroleum Distillate; See 85 15 Note 1.

N orE 1.-1he petroleum distillate was a paratnic hydrocarbon fraction bo-'Iing in the temperature range between 130 and 140 C.

Norn 2,-The distillate was heterogeneous and was estimated to be about 15 vol. percent isopropanolamine layer and about 85 vol. percent hydrocarbon layer.

EXAMPLE 2 This example shows a continuous method for separating an alkanolamine from a piperazine compound by azeotropic distillation.

A mixture comprising trans-2,5dimethylpiperazine and about 2O percent by weight isopropanolamine is fed continuously to a mid-point in an eicient fractional distillation column operating at atmospheric pressure. The column also contains a paraiinic hydrocarbon codistillation agent which is a petroleum fraction having a normal boiling range from 130 to 140 C. The column is heated by a bottom reboiler. The vapors fractionated out at the top of the column consist of the codistillation agent and isopropanolamine and are substantially free of dimethylpiperazine. The top vapors are condensed and separate into two layers, one consisting principally of isopropanolamine and the other consisting principally of the codistillation agent. The latter, i.e. the codistillation agent, is continuously returned to the column as reflux while the isopropanolatrune layer is withdrawn. At a point in the lower portion of the column, the temperature is continuously observed and held to about 145 C. When the temperature at the control point rises above the set value, it is indicative that too little codistillation agent is present in the column, and corrective steps, such as increasing the reflux of codistillation agent, are taken. When the temperature at the control point falls below the set value, it is indicative that too much codistillation agent remains in the residue, and corrective steps, such as increasing the Thereby, the column residue. is substantially freed of both isopropanolamine and codistillation agent and is continuously withdrawn.

EXAMPLE 3 This example shows a continuous method for separat- EXAMPLE 4 This example demonstrates the use of a codistillation agent, ethylbenzene, for the separation of isopropanolamine from 2,5-dimethylpyrazine.

In a mixture of isopropanolamine and 2,5-dimethylpyrazine, the latter is the more volatile but the mixture is dicult to separate by fractional distillation, as is Vshown by the following test.

A 65-gram portion of a mixture consisting of 57.1 percent by weight isopropanolamine and 42.9 percent 2,5- dimethylpyrazine was charged to a still having a 5%: by 12-inch distillation column packed with'glass helices. 'I'he column had a fractionatng eicien'cy of about four theoretical plates. A distillate fraction of 9.5 grams was taken at distillation temperatures between 1489 and 152 C. and contained 57.6 percent by weight 2-,5-dimethylpyrazine.

In contrast, a l0-gram portion of that same starting mixture, together with 70 grams of ethylbenzene, was distilled in the same still under the same conditions. Two successive distillate fractions, totaling 33 grams and taken at distillation temperatures between and 131 C percent by weight ethyl- 19.3percent isopropanolamine andy only a trace contained an average of 80.6 benzene,

. prima of 2,5-dirnethjtflpyrazine.y Isopropanolamine is more vol'- This 'example illustrates the separation of mixtures of 2,5dimethylpyrazine and relatively small proportions of isopropanolamiue by codistillationrwith ethylbenzene.

-To a mixture of49 grams 2,5diniethylpyrazine and 20 gramsisopropanolamine was added 90 grams ethylbenzeneand'the" resulting mixture'was distilled in the still described in'Example 4. The distillation was continued to a distillation temperatureY of' 142 C. Sub` stantially all of the isopropanolamine was obtained in the distillate and the residue was 95 percent 2,5-dimethyl- EXAMPLE 6 l Vapor-liquid'equilibrium data were obtained for a mixture of isopropanolamine and cis-2,5-dirnethylpiperrazine alone and in the presence of various codistillation agents. in Table II'the codistillation agents are identified and the compositions of the vapor-'and of the liquid at equilibrium at atmospheric pressure are shown for each test. The compositions Vare shown in terms of the weight percent concentration of isopropanolarnine on an agent-V free basis, i.e., the proportion of isopropanolamine based on the combined lweights of isopropanolamine and' cis-rV if 1. A method of separatinga lower alkanolamine from 'a mixtureY thereof with a pyrazine selected from the class consisting of pyrazine and lower alkyl pyrazines which comprises adding to the mixture a volatile liquid having Ya. normal boiling point not higher than 200" C. selected from the class consisting of saturated aliphatic hydrocarbons having at least eight carbon atoms, lower alkylbenzenes, and chlorobenzene, heating the mixture and distilling` therefrom an azeotrope consisting predominantly of the lower alkanolamine and said volatile liquid.

' 2. A method of separating isopropanolamine from a mixture thereof with 2,5-dmethylpyrazine which comprises adding to the mixture a volatile liquid having a normal boiling point not higher than 200 C. selected from the class consisting of saturated aliphatic hydrocarbons having at leasteight carbon atoms, lower alkylbenzenes, andy chlorobenzene, heating the mixture and distilling therefrom an azeotrope consisting predominantly of isopropanolamine and said volatile liquid. e

3. A vmethod of separating'Visopropanolamine from a e mixture thereof with 2,5-dimethylpyrazine which comprises adding to the mixture a volatile liquid saturated aliphatic hydrocarbon having at least eight carbon atoms and yliquid aliphatic hydrocarbon.

a normal boiling point not higher than 200 C., heating the mixture and distilling therefrom an azeotrope consisting predominantly of isopropanolamine and said volatile 4. A method of separating isopropanolamine from a mixture thereof with 2,5-dimethylpiperazine which comprises adding to the mixture a volatile liquid saturated aliphatic hydrocarbon having at least eight carbon atoms and a normal boiling point not higher than 200 C., heating the mixture and distilling therefrom andk azeotrope consisting predominantly Vof isopropanolamine and said volatile liquidgaliphatic hydrocarbon.

References Cited inthe tile of this patent UNITED STATES PATENTS 2,940,933 Patton Dec. 2, 1953 OTHER REFERENCES Fowler Nov. 26, 1956` 

1. A METHOD OF SEPARATING A LOWER ALKANOLAMINE FROM A MIXTURE THEREOF WITH A PYRAZINE SELECTED FROM THE CLASS CONSISTING OF PYRAZINE AND LOWER ALKYL PYRAZINES WHICH COMPRISES ADDING TO THE MIXTURE A VOLATILE LIQUID HAVING A NORMAL BOILING POINT NOT HIGHER THAN 200* C. SELECTED FROM THE CLASS CONSISTING OF SATURATED ALIPHATIC HYDROCARBONS HAVING AT LEAST EIGHT CARBON ATOMS, LOWER ALKYLBENZENES, AND CHLOROBENZENE, HEATING THE MIXTURE AND DISTILLING THEREFROM AN AZEOTROPE CONSISTING PREDOMINANTLY OF THE LOWER ALKANOLAMINE AND SAID VOLATILE LIQUID. 